Heat exchangers that transfer heat from one fluid flow to another fluid flow and wherein the mass flow rates of each of the fluid flows are substantially proportional to each other are known. Perhaps the most common example of such heat exchangers are recuperative heat exchangers that recoup or recover useful heat from the working fluid of a system as the working fluid flows through the system. One example of such a recuperative heat exchanger can be found in the fuel processing system of proton exchange membrane (PEM) type fuel cell systems. In typical fuel cell systems, a fuel, such as methane or a similar hydrocarbon, is used as the source of hydrogen for the fuel cell. This hydrocarbon must be reformed within the system prior to reaching the fuel cell in order to provide the hydrogen gas. Reforming is typically carried out by a fuel processing system through a series of catalyst-aided chemical reactions, all of which need to occur within different distinct temperature ranges. Heat exchangers, including recuperative heat exchangers, are used to alternately heat and cool the gas stream to the desired catalytic reaction temperatures for the process.
One example of such fuel cell systems is shown in FIG. 1. The PEM fuel cell system 8 of FIG. 1 utilizes methane (CH4) as its fuel and includes a proton exchange membrane fuel cell 10, an anode tail gas oxidizer 20, a heat exchanger 22 that transfers heat from the tail gas to the air/methane and water (H2O) entering the system 8, a humidifier 24 that humidifies the humidified air/methane mixture from the heat exchanger 22, an auto-thermal reformer (ATR) 26, a high temperature water-gas shift reactor (HTS) 28 which is sometimes incorporated into the ATR 26, a recuperative heat exchanger 30 that transfers heat from the reformate produced by the ATR 26 to the humidified air/methane mixture from the humidifier 24, another water-gas shift reactor which in the illustrated example is a low temperature water-gas shift reactor (LTS) 32, and a preferential oxidizer (PROX) 34. Optionally, several other heat exchangers 36 can be added at various locations in the fuel cell system 8 to transfer heat between the various components of the fuel cell system 8. The heat exchanger 22, the humidifier 24, the recuperative heat exchanger 30, the ATR 26, the HTS 28, the LTS 32, and the PROX 34 form a fuel processing system 36 for the fuel cell system 8. It will be understood by those skilled in the art that such PEM fuel cell systems also include a cathode gas flow to the fuel cell 10, as well as the components associated with the cathode gas flow, none of which are shown in FIG. 1. It will also be understood by those skilled in the art that some fuel cell systems may incorporate a mid-temperature water-gas shift reactor in place of the LTS 32 or the HTS 28, or both.
Typically, the catalytic reaction in the ATR 26 requires an inlet gas temperature of about 500° C. to about 750° C. with a preferred temperature of approximately 630° C. The catalytic reaction in the LTS 32 requires the inlet gas flow to have a temperature in the range of about 180° C. to about 240° C. with a preferable target temperature of approximately 210° C. Because the catalytic reactions in the ATR 26 and the LTS 32 require the temperature of the incoming gas flow to be within a relatively narrow temperature range, the control of the outlet temperatures from the recuperative heat exchanger 30 is critical to the operation of the fuel cell system 8. However, as the electrical load on the fuel cell system is varied, the flow of gas through the system, including the heat exchanger 30, likewise varies, sometimes in the range of 10 to 1. Typically, the heat transfer effectiveness of the heat exchanger 30 will not be constant for a widely varying mass flow rate of the gas flow there through, and the gas temperatures exiting the heat exchangers will therefore not be held within the desired temperature range at all flow rates, unless some sort of control system is incorporated with the heat exchanger 30. The typical solution to this problem is to actively control the amount of flow which passes through the heat exchanger 30 via a bypass control system such as a feed back/bypass control system 38 as shown in FIG. 1. The control system 38 typically includes a temperature sensor 40 that monitors the temperature of the reformate gas flow exiting the heat exchanger 30 and provides the monitored temperature to a proportional-integral-derivative (PID) controller 42 that compares the monitored temperature with a set point temperature and continually adjusts a solenoid-controlled bypass valve 44 to shunt a portion of the humidified air/methane mixture around the heat exchanger 30 via a bypass flow path 46. This limits the amount of heat that can be transferred between the flow streams through the heat exchanger 30 and prevents over cooling of the reformate to the LTS 32.
While systems typified by the one described above may perform satisfactory for their intended purpose, there is always room for improvement. For example, the use of an active control system may add cost and complexity to such systems, while reducing the reliability of such systems.